Only run [declareOutputs] once for each asset/transformer pair.

pkg/barback/test/transformer/declaring_bad.dart

Index: pkg/barback/test/transformer/declaring_bad.dart
diff --git a/pkg/barback/test/transformer/declaring_bad.dart b/pkg/barback/test/transformer/declaring_bad.dart
new file mode 100644
index 0000000000000000000000000000000000000000..41804ce766b1e775f52f8773a5e63fc8ccfe926f
--- /dev/null
+++ b/pkg/barback/test/transformer/declaring_bad.dart
@@ -0,0 +1,50 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library barback.test.transformer.declaring_bad;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
+
+import 'bad.dart';
+import 'mock.dart';
+
+/// A transformer that throws an exception when run, after generating the
+/// given outputs.
+class DeclaringBadTransformer extends MockTransformer
+    implements DeclaringTransformer {
+  /// Whether this should throw an error in [declareOutputs].
+  final bool declareError;
+
+  /// Whether this should throw an error in [apply].
+  final bool applyError;
+
+  /// The id of the output asset to emit.
+  final AssetId output;
+
+  DeclaringBadTransformer(String output, {bool declareError: true,
+          bool applyError: false})
+      : this.output = new AssetId.parse(output),
+        this.declareError = declareError,
+        this.applyError = applyError;
+
+  Future<bool> doIsPrimary(AssetId id) => new Future.value(true);
+
+  Future doApply(Transform transform) {
+    return newFuture(() {
+      transform.addOutput(new Asset.fromString(output, "bad out"));
+      if (applyError) throw BadTransformer.ERROR;
+    });
+  }
+
+  Future declareOutputs(DeclaringTransform transform) {
+    return newFuture(() {
+      if (consumePrimary) transform.consumePrimary();
+      transform.declareOutput(output);
+      if (declareError) throw BadTransformer.ERROR;
+    });
+  }
+}